303
I used 303 on my PVC dinghy and yet it still broke down from the UV-abc rays and went gluey, still serviceable but the surface has started to deteriorate after just a few years. I sponged that 303 on liberally every summer and only had the dinghy exposed two months max per year.
I do use that Simonize Black shine tire protectant on my rub rails though (every year) and they are holding up just fine so I think I'll try it on the rubber boot that the throttle cables pass through. It is a bit messy though and does leave a residue for a while but it seems to be working well on my rub rail, thank goodness.